Nowadays, from the viewpoint of global environment conservation, there is a strong demand for increasing the fuel efficiency of automobiles. Therefore, since there is a strong demand for reducing the weight of automobile bodies, particularly in the case of automotive parts, consideration is being given to increasing the strength of a steel sheet, which is a raw material for the parts. However, since formability generally decreases with an increase in the strength of a steel sheet, in the case where parts are manufactured from a high-strength steel sheet, there are problems in manufacture processes, for example, it is difficult to perform forming or shape fixability decreases.
Therefore, in response to such problems, techniques for manufacturing high-strength automotive parts and the like have been put into practice by performing a hot pressing process on a steel sheet. In a hot pressing process, a steel sheet is heated to a temperature in a temperature range in which austenite is formed, then transported to a pressing machine, and then rapidly cooled while being formed into a part having a desired shape by using a forming tool in the pressing machine. In this cooling (rapid cooling) process in the forming tool, since the microstructure of the part transforms from an austenite phase to a martensite phase, it is possible to obtain a part having not only the desired shape but also high strength.
In addition, nowadays, from the viewpoint of achieving occupant safety, there is a demand for increasing the crashworthiness of the automotive parts. Since it is thought that manufacturing a part having high uniform elongation is effective for increasing the crashworthiness of the parts, from the viewpoint of increasing the capability (impact-energy-absorbing capability) of absorbing energy at the time of a collision, there is a strong demand for a hot-pressed part having not only a high strength but also excellent uniform elongation.
In response to such a demand, for example, Patent Literature 1 proposes a hot-pressed product which is obtained by forming a steel sheet by using a hot press forming method. The hot-pressed product described in Patent Literature 1 has a chemical composition containing, by mass %, C: 0.15% to 0.35%, Si: 0.5% to 3%, Mn: 0.5% to 2%, Al: 0.01% to 0.1%, Cr: 0.01% to 1%, B: 0.0002% to 0.01%, Ti: (the N content)×4 to 0.1%, and N: 0.001% to 0.01% and a microstructure including, in terms of area fraction, martensite: 80% to 97%, retained austenite: 3% to 20%, and the remainder: 5% or less. Patent Literature 1 states that, according to the technique disclosed in Patent Literature 1, since it is possible to form a metallographic structure in which an appropriate amount of retained austenite is retained, it is possible to manufacture a hot-pressed part having increased ductility (residual ductility) inherent in the product.
In addition, Patent Literature 2 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 2 has a chemical composition containing, by mass %, C: 0.15% to 0.30%, Si: 0.05% to 3.0%, Mn: 1.0% to 4.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a ferrite phase to the whole microstructure is 5% to 65% in terms of area fraction, in which the area fraction of a martensite phase is 35% to 95%, and in which the average grain diameter of a ferrite phase and a martensite phase is 7 μm or less, high strength represented by a tensile strength TS of 1300 MPa to 1450 MPa, and excellent ductility represented by an elongation El of 8% or more.
In addition, Patent Literature 3 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 3 has a chemical composition containing, by mass %, C: 0.20% to 0.40%, Si: 0.05% to 3.0%, Mn: 1.0% to 4.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a ferrite phase to the whole microstructure is 5% to 55% in terms of area fraction, in which the area fraction of a martensite phase is 45% to 95%, and in which the average grain diameter of a ferrite phase and a martensite phase is 7 μm or less, high strength represented by a tensile strength TS of 1470 MPa to 1750 MPa, and excellent ductility represented by an elongation El of 8% or more.
In addition, Patent Literature 4 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 4 has a chemical composition containing, by mass %, C: 0.30% to 0.45%, Si: 0.05% to 3.0%, Mn: 1.0% to 4.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a ferrite phase to the whole microstructure is 5% to 40% in terms of area fraction, in which the area fraction of a martensite phase is 60% to 95%, and in which the average grain diameter of a ferrite phase and a martensite phase is 7 μm or less, high strength represented by a tensile strength TS of 1770 MPa to 1940 MPa, and excellent ductility represented by a total elongation El of 8% or more.
In addition, Patent Literature 5 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 5 has a chemical composition containing, by mass %, C: 0.35% to 0.50%, Si: 0.05% to 3.0%, Mn: 1.0% to 4.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a ferrite phase to the whole microstructure is 5% to 35% in terms of area fraction, in which the area fraction of a martensite phase is 65% to 95%, and in which the average grain diameter of a ferrite phase and a martensite phase is 7 μm or less, high strength represented by a tensile strength TS of 1960 MPa to 2130 MPa, and excellent ductility represented by an elongation El of 8% or more.
In addition, Patent Literature 6 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 6 has a chemical composition containing, by mass %, C: 0.18% to 0.21%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a martensite phase to the whole microstructure is 90% to 100% in terms of area fraction, and in which the average grain diameter of prior austenite grains is 8 m or less, high strength represented by a tensile strength TS of 1300 MPa to 1450 MPa, and excellent ductility represented by an elongation El of about 10.0% to 14%.
In addition, Patent Literature 7 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 7 has a chemical composition containing, by mass %, C: 0.22% to 0.29%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a martensite phase to the whole microstructure is 90% to 100% in terms of area fraction, and in which the average grain diameter of prior austenite grains is 8 μm or less, high strength represented by a tensile strength TS of 1470 MPa to 1750 MPa, and excellent ductility represented by an elongation El of about 9.5% to 12%.
In addition, Patent Literature 8 proposes a hot-pressed part having excellent ductility. The hot-pressed part described in Patent Literature 8 has a chemical composition containing, by mass %, C: 0.30% to 0.34%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a martensite phase to the whole microstructure is 90% to 100% in terms of area fraction, and in which the average grain diameter of prior austenite grains is 8 μm or less, high strength represented by a tensile strength TS of 1770 MPa to 1940 MPa, and excellent ductility represented by an elongation El of about 8.0% to 11%.
In addition, Patent Literature 9 proposes a hot-pressed part having excellent ductility. The hot-pressed member described in Patent Literature 9 has a chemical composition containing, by mass %, C: 0.35% to 0.40%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, Al: 0.005% to 0.1%, and N: 0.01% or less, a microstructure, in which the proportion of a martensite phase to the whole microstructure is 90% to 100% in terms of area fraction, and in which the average grain diameter of prior austenite grains is 8 μm or less, high strength represented by a tensile strength TS of 1960 MPa to 2130 MPa, and excellent ductility represented by an elongation El of about 8.0% to 10%.
In addition, Patent Literature 10 proposes a high-strength hot-pressed part which is obtained by performing hot pressing. The high-strength hot-pressed part described in Patent Literature 10 has a chemical composition containing, by mass %, C: 0.12% to 0.69%, Si: 3.0% or less, Mn: 0.5% to 3.0%, Al: 3.0% or less, and N: 0.010% or less, in which the relationship Si+Al≥0.7% is satisfied, a microstructure including martensite and bainite containing retained austenite and bainitic ferrite, in which the proportion of martensite to the whole microstructure is 10% to 85% in terms of area fraction, in which tempered martensite constitutes 25% or more of martensite, in which the proportion of retained austenite is 5% to 40%, in which the proportion of bainitic ferrite among bainite to the whole microstructure is 5% or more in terms of area fraction, in which the sum of the proportions of martensite, retained austenite, and bainitic ferrite to the whole microstructure is 65% or more in terms of area fraction, and in which the average C content in retained austenite is 0.65% or more, a tensile strength TS of 980 MPa or more, and excellent ductility represented by a (TS×El) of 17000 MPa % or more.